The need for a communication service which is convenient, affordable, and reliable has been fueled by market demand. The convenience requirement is covered by providing the customer with portable wireless communication devices. To date, wireless portable communications devices have primarily consisted of wireless portable telephones (cellular) for wireless voice communications. Recent market trends are gravitating towards providing integrated wireless voice and data communications (fax, e-mail, etc.) and making it available "on the move/on demand." Affordability is achieved through the economies of scale from the ever increasing number of users of these technologies and the resulting decrease in the manufacturing costs of integrated electronics. Lastly, the issue of reliability of wireless communications is the topic of the present invention; more specifically, the ability of wireless communications receiver systems to better reject interfering noise in wireless communications.
In referring to noise in what follows, one understands its definition in the context of electrical noise with respect to an electrical system under consideration. Electrical noise thus defined can take a variety of forms, including: radio frequency (RF) noise, thermally induced electrical noise and signal distortions introduced by electrical components in performing their intended function. Thermally induced electrical noise and signal distortion introduced by electrical components can be characterized and establishes a lower threshold for the transmission and detection of radio frequency signals.
RF noise comprises RF signals transmitted by electrical equipment as a side effect of their intended function, RF signals of a natural source which create a background level, and RF signals transmitted by other electrical equipment as a direct effect of its intended function but which interferes with the operation of a system under consideration. The first form of RF noise can be handled by careful equipment design and proper shielding and is usually mandated through government regulation. The second form of RF noise can be characterized and further degrades the lower threshold for transmission and detection of RP signals of interest. The last form of RF noise should not normally constitute a problem due to careful government control and allocation of the scarce radio spectrum. This last form of RF noise encompasses signal jamming, whether it is intentional or otherwise. It is this last type of RF noise that is characterized as RF interference. The present invention attempts to isolate RF interference and to alleviate its effects.
In wireless communications there are many levels at which noise can disturb a wireless link and therefore introduce signal degradation. RF transmissions for example, are subject to signal distortion due to the severe nature of the propagation medium. There are many types of signal distortions and generally they are characterized in order to design wireless communications systems that are immune to their effects. One such signal distortion is a type of self-interference that arises from the reception of multiple reflections of the same signal. These multiple reflections cause attenuation of the received signal and in a digital system induce a delay spread that tends to smear the bits comprising the digitized voice. This is known as multipath fading. Although multipath fading is a frequency dependant phenomenon that places severe constraints on the ability to provide a reliable wireless link, it can nevertheless be characterized and its affects minimized through conservative design (i.e., providing adequate fade margin) and by employing such techniques as spatial diversity.
One of the most recent techniques to be employed in commercial wireless applications, one which is inherently resilient to multipath fading, employs a modulation technique which encodes the digital sequence of bits to be transmitted in such a way that the resulting signal spectrum is spread over a much wider range of frequencies than is necessary to transmit the information. This technique is referred to as direct-sequence spread-spectrum or DSSS.
The Code Division Multiple Access (CDMA) digital cellular standard, which is based upon DSSS techniques, is arguably one of the most robust and potentially highest capacity systems yet deployed. However, even with its theoretical ability to suppress interference due to its "processing gain" (a function of the ratio of the RF bandwidth of the DSSS signal to the information signal bandwidth) it is nevertheless still susceptible to and is fundamentally an interference limited system.
RF interference which is characterized as being high-power (i.e., strong amplitude) and narrow bandwidth, is a form of intra-system interference in that it is produced by other concurrent users, either legal or illegal, who are transmitting in the same spectrum allocated for the CDMA service. This can lead to severe degradation of the DSSS-based CDMA system in terms of capacity (i.e., number of users), voice quality, etc. In some cases it can result in the complete loss of wireless communications. Under such conditions, this narrowband interference or NBI is said to overwhelm the DSSS receiver.
The present invention is concerned with a system for the detection, isolation and cancellation of NBI that falls within the bandwidth of a DSSS signal for the purpose of restoring the DSSS receiver to an acceptable operational state and thereby render it immune to NBI.